The present invention relates to an apparatus for growing a crystal, used for manufacture of a silicon single crystal or the like used as a semiconductor material.
A variety of methods are available for manufacturing silicon single crystals, of which the Czochralski method (the CZ method) is typical. In producing a silicon single crystal by the CZ method, as is well known, a seed crystal is immersed in a silicon melt formed in a quartz crucible. The seed crystal is gradually pulled to allow a silicon single crystal to grow beneath the seed crystal while rotating the crucible and seed crystal.
In the pulling of silicon single crystal by such CZ method, it is known that the defect distribution or the like in a crystal section are governed by the rate of crystal growth, that is the pulling rate. More specifically explained, as the pulling rate is increased, a ring OSF generating area is moved towards the periphery and is finally excluded to the outside of the effective portion of the crystal. Conversely, a decrease in the pulling rate drifts a ring OSF generating area towards the central part of the crystal, and eventually the zone disappears in the central part.
While both the outside and inside of an OSF generating area are defect generating areas, their kinds of defects are different. In addition, it is known that a significant increase of the pulling rate, as a matter of course, improves productivity while refining the defects. Consequently, a speed-up in the pulling has been pursued as an approach to growing crystals.
Provision of a heat shield is known as a technique for high speed pulling. A heat shield is a cylindrical heat shielding member in a shape of inverted truncated cone that is disposed surrounding the single crystal. The shield is provided to speed up the withdrawal by shielding the radiation heat primarily from the melt in the crucible and heaters placed outside the crucible to facilitate cooling of the single crystal to be pulled from the melt.
Furthermore, attention is recently given to a technique where a coller that is forcedly cooled by water is placed inside the heat shield (Japanese Patent Laid-Open Nos. 63-256593, 8-239291, 11-92272 and 11-292684). Installation of a coller that is forcedly cooled by water inside the heat shield surrounding the single crystal facilitates cooling of the single crystal, particularly the high temperature portion thereof, leading to a further speedup of the pulling.
However, in an apparatus for growing a crystal that uses a cooler, proposed thus far, the cylindrical main body of the cooler thereof is supported and fixed inside the body of the crystal puller using part of the cooler itself, i.e. a flange integrated with the cylindrical main body or water piping projected from the cylindrical main body as a supporting member.
Use of a copper-based metal member forcedly cooled by passage of water is advisable as a cooler from the viewpoint of cooling ability for a single crystal. However, in a structure integrating a cylindrical main body with a supporting member thereof as discussed above, when the cylindrical main body is made of a copper-based metal, part of the supporting member also essentially needs to be composed of a copper-based metal as in the main body.
Studies of the present inventors have found out that because such a cooler frequently uses a relatively expensive copper-based metal, it not only increases the cost of manufacture, but also lowers the dislocation free pulling rate in high speed pulling due to excessive cooling of part of a supporting member. This seems to be because the construction of part of a supporting member using a copper-based metal the same as that of the main body over-cools part of the supporting member, which causes precipitation of a large amount of silicon oxide or the like on the surface thereof, to be dropped, leading to generaten of dislocation in a single crystal during pulling.
In addition, composing part of supporting member of a copper-based metal the same as that of the main body makes it difficult to keep the mechanical strength thereof. This may lead to risks that the distortion of part of a supporting member makes the position of the main body deviate to thereby cause the distortion of a pulled crystal resulting in the prevention of speedup, or brings about a stream explosion due to damage of piping. These risks are increased particularly when copper-based water piping is used as a supporting member.
Further, concerns also rise that the main body is difficult to design in accordance with pulling conditions resulting in a failure to sufficiently increase the pulling rate because the main body cannot be taken out of the supporting member.
An object of the present invention is to provide an apparatus for growing a crystal, the apparatus capable of avoiding a decrease in dislocation free pulling rate without lowering cooling capacity for a crystal being pulled, capable of decreasing the manufacture cost of a cooler, and capable of solving a variety of problems related to deviation in the position of a cooler.
In order to achieve the above-mentioned object, the present invention provides an apparatus for growing a crystal in which a coller is disposed so as to surround a single crystal to be grown from a raw material melt, and the cooler is attachably and detachably placed on the body of the above-mentioned crystal puller by an independent supporting member separated from the cooler, in an apparatus for growing a crystal that produces a single crystal from a raw material melt in a crystal puller by the CZ method.
For the placement of a coller on the body of the puller, the two can be composed of different materials by utilizing an independent supporting member separated from the cooler. More specifically, a coller can be made from a copper-based metal of good thermal conductivity; a supporting member can be composed of a material, such as stainless steel, which is cheaper than copper-based metals, is high in mechanical strength and is inferior in thermal conductivity.
Doing so can prevent excessive cooling of the supporting member without lowering cooling capacity for a crystal being pulled, avoiding a decline in dislocation free pulling rate. In addition, the cost of manufacture of a cooler is decreased and the support strength of the cooler is increased, which solves various problems related to deviation in the position thereof. Moreover, making a cooler detachable from the supporting member facilitates designing of a cooler in accordance with pulling conditions.
A cooler is composed of a metal member forcedly cooled by passage of water. The metal is preferably a copper-based metal that contains copper as the primary component and exhibits good thermal conductivity. The specific size and shape of a cooler is designed, as necessary, depending on pulling conditions.
A supporting member preferably has a plurality of supporting arms that are radially disposed around the pulling axis in the body of a puller because the arms do not prevent gas flow in the puller and are cost effective in material. Constituent materials of a supporting member are preferably those that are cheaper than copper-based metals, high in mechanical strength and low in thermal conductivity. More specifically, stainless steel is the most suitable; however, graphite, carbon fiber composites, and the like are effective as well. It is preferable in terms of speed-up of pulling rate that the supporting member is made from a material of which thermal conductivity is inferior to those of copper-based metals and then is water cooled.
A method of fixing a cooler and a supporting member thereof is not particularly limited; however, coupling with bolts is simple and preferable. More simply, although a cooler can simply be put, or hung on a supporting member thereof, it is important not to put a load on water piping to the cooler.
A cooler is desirably combined with a heat-shielding member and placed therein. Combination with a heat-shielding member not only facilitates cooling of a crystal, but also effectively suppresses a rise in the temperature of the cooler itself, leading to a promotion in speed-up of pulling rate.